1. Field of the Invention
The present invention relates to a device or, respectively, a system, in particular a printer or copier, with two electrographic printing units of substantially the same type, wherein each of the printing units prints an image pattern on a sheet-type material. The device includes an input section via which the sheet-type material can be supplied individually in successive fashion and an output section via which the printed sheet-type material is ejected individually in successive fashion.
2. Description of the Prior Art
Electrophotographic printers of this type are known, for example, from DE 34 16 252 A1, EP 0 104 022 A2, DE-PS 1,280,605, DE 34 07 847 A1 or WO 91/13386. With printers of this type, high printing performance can be achieved, even in duplex operation, as long as both printing units are working as they should. However, the devices specified in the cited publications have the disadvantage that, in the case of failure of one of the printing units, a drastic reduction in the performance of the duplex operation results. This is because a sheet that is to be printed on both sides in duplex operation must then be accelerated repeatedly in the opposite direction in order to enable it to be printed on both sides by the one remaining printing unit.
From U.S. Pat. No. 5,150,167, a printing unit is known that contains two printing units. One of the printing units has a priority status. As such, sheets that have been printed in the lower-order printing unit additionally can be printed in the priority printing unit. However, if the priority printing unit fails, double printing and, in particular, duplex printing is not possible.
From U.S. Pat. No. 5,208,640, a further print apparatus is known that contains several printing units. The printing units are connected with one another via a ring system, wherein a common turning apparatus with a shunt for all printing units is used. If the turning apparatus fails, e.g. due to a switching error in the shunt, duplex operation is no longer possible.
An object of the present invention, therefore, is to provide a device, in particular a printer or a copier, that has a high print performance or, respectively, copier performance, in particular in duplex operation.